Iron-nickel-chromium alloys, as is known, are extensively used under diverse service conditions requiring any number of different metallurgical properties. Such materials offer various degrees of corrosion resistance, ductility, stress-rupture strength, etc. One of the more demanding in-service applications involves the petrochemical industry wherein natural gas liquid feedstocks used in olefin pyrolysis are experienced. This environment is causative of rather severe degradation in respect of alloys currently used for radiant section tubes of pyrolysis furnaces. In an article co-authored by D. E. Hendrix and M. W. Clark entitled "Contributing Factors To the Unusual Creep Growth Of Furnace Tubing In Ethylene Pyrolysis Service and presented at the Mar. 25-29, 1985 International Corrosion Form, the writers described how currently used alloys HK-40 (Nominally 25% Cr, 20% Ni, Bal. Fe) and HP-40+1% Nb (nominally 25% Cr, 35% Ni, 1% Nb Bal. Fe) undergo premature failure due to carburization attack which, in turn, leads to excessive axial creep growth in respect of the pyrolysis heater tubing formed from such alloys. (Carburization is a phenomenon by which the alloy structure is environmentally degraded from the surface inward. As a consequence, the load bearing capacity of an alloy is adversely impacted as reflected by impaired strength, particularly stress-rupture and creep, and reduced ductility. Initial attack is usually along the grain boundaries and this tends to accelerate failure).
Apart from (i) carburization resistance and (ii) stress-rupture strength what is also desirable for ethylene pyrolysis tubing is an alloy which is (iii) highly oxidation resistant, (iv) both hot and (v) cold workable such that mill product forms can be readily produced without deleterious cracking, (vi) ductile, (vii) structurally stable, i.e., will resist forming detrimental quantities of undesirable phases such as sigma, and (viii) weldable. For an alloy to be highly carburization and creep resistant, for example, but not workable is self-defeating since an alloy in wrought form could not be produced. Conversely, to be workable without high resistance to carburizing attack would not be a panacea.